In the packaging of food products with thermoplastic films, a widely used oxygen and moisture barrier is vinylidene chloride copolymers as for example manufactured with vinyl chloride or methyl acrylate comonomer. For example, a commonly used film is a multilayer heat shrinkable type including a vinylidene chloride-vinyl chloride copolymer barrier layer with a polyolefin such as ethylene vinyl acetate (EVA) laminated to one side of the barrier layer to form a heat sealable layer. An abuse-resistant layer is also frequently employed and laminated to the opposite side of the barrier. This layer may also be formed of a polyolefin as for example EVA. The resulting three layer film is biaxially stretched if heat shrinking capability is desired, and fabricated into a bag with the heat sealable layer on the inside for direct contact with the food product and the abuse-resistant layer on the outside.
After inserting the food product such as fresh red meat or processed meat in the bag, the latter may be evacuated to pull the film inside surface against the meat product outer surface, but in any event the bag open end is sealed, most commonly by the application of heat, i.e., a heat seal. In some instances the food product is also cooked in the sealed bag by immersion in a hot water bath, i.e. a "cook-in" system.
It is also common to irradiate the film before fabrication into a bag, and if multilayered this effects cross-linking of the various layers to different degrees, depending on the layer composition and proximity to the irradiation source. There are two main reasons for irradiation: to strengthen the thermoplastic film's physical properties such as impact strength and tensile strength, and also to raise the thermoplastic film's softening point. This in turn widens the heat sealing range thereby permitting operation of the heat sealing device at higher temperatures.
It is known that as a result of irradiation, odors develop in thermoplastic bags at least in part formed from vinylidene chloride copolymers. It has also been determined that in a three layer irradiated film of the polyolefin/vinylidene chloride copolymer/polyolefin type, the odors come primarily from the vinylidene chloride copolymer. Although this is not a problem in most food packages because the odors are relatively mild and do not permeate the food itself, there are sporadic complaints about odors from the food processors.
The odors are believed derived from compounds directly or indirectly formed from partial decomposition of the vinylidene chloride copolymer as a result of irradiation. For example, this material is thermally unstable and when heated by irradiation evolves hydrochloric acid or chloride in the salt form. This leaves a carbonaceous residue. The reaction is quite complicated and affected by physical changes in the solid, the method of preparing the vinylidene chloride copolymer and its purity. The polymer discolors gradually from white to yellow, and under extreme conditions it turns brown and eventually black.
Analysis of the odorous components in multilayer films of the vinylidene chloride copolymer core-barrier layer type indicates the presence of inorganic and organic acids, alcohols, ketones, esters and aldehydes of various types and amounts. Most of the identified chemical species have been determined to evolve from the vinylidene chloride copolymer barrier layer.
The prior art literature contains many theories on the source of these compounds, but it is generally recognized that the aforementioned HCL (as a decomposition by product of vinylidene chloride copolymer decomposition) is the most important reactant in the decomposition mechanism. If the multilayer film contains ethylene vinyl acetate as at least one of the other layers, acetic acid results from this irradiation. A myriad of chemical compounds are believed formed from the free radical combination and breakdown of the additives in the film as for example processing aids. Hydrochloric acid reacts with at least some of the chemical specie to produce at least some of the odor producing compounds.
The stability of vinylidene chloride copolymers also depends on the comonomer. Copolymers with vinyl chloride and the acrylates degrade slowly. Acrylonitrile copolymers degrade more rapidly and release HCN as well as HCL.
The art of stabilizing vinylidene chloride copolymers is highly developed. It was believed that the ideal stabilizer system should achieve absorb or combine with evolved HCL irreversibly under conditions of use, but not strip HCL from the polymer chain, and possibly possess antioxidant activity so as to prevent the formation of carbonyl groups and other chloride-activating structures.
A wide variety of commercially available compounds known for effectiveness as deodorants in polyvinylidene chloride copolymer systems have been tested but none have been satisfactory. In particular, acid acceptor style compounds were tried with disappointing results. These compounds included epoxidized soy bean oil, sodium citrate and tetra sodium pyrophosphate.
Another well-known hydrochloric acid absorbent is hydrotalcite. It is known as a mineral having a chemical structure of the formula Mg.sub.4.5 Al.sub.2 (OH).sub.13 CO.sub.3 3.5H.sub.2 O which has been naturally produced in only very few limited areas such as Suarum, Norway and the Ural mountains. Hydrotalcite is also synthesized, manufactured and sold by Kyowa Chemical Industry Co., Ltd of Tokyo, Japan. As used herein, "hydrotalcite" means the above described compound irrespective of its source, i.e., naturally occurring or synthetically prepared. A process for manufacturing hydrotalcite is for example described in Kumura et al U.S. Pat. No. 3,539,306, incorporated herein to the extent pertinent. In general, this process involves mixing (A) an aluminum component selected from aluminum hydroxide, aluminum amino acid salts, aluminum alcoholate, water-soluble aluminates, aluminum nitrate and aluminum sulphate, with (B) a selected magnesium component and (C) an alkali carbonate. The magnesium component is selected from magnesium oxide, magnesium hydroxide and water-soluble magnesium salts. The mixing is performed in an aqueous medium at temperatures of from 0.degree. to 150.degree. C. at a pH of at least 8. The mixture has a molar ratio in terms of Al.sub.2 O.sub.3 : MgO of substantially 1:6. The hydrotalcite is obtained in the form of precipitate, washed with water if desired, and thereafter the solid is separated by known solid-liquid separation means such as centrifuge, followed by drying to serve as the dry product.
According to Kumura et al, the critical feature of the hydrotalcite manufacturing process is that the reaction of the aluminum component, magnesium component and carbon dioxide is performed in water and under basic conditions. It is thereby possible to provide carbon dioxide in the form of carbonate ion, and consequently mild reaction conditions with respect to temperature and pressure are said to be feasible.
Applicants have followed the prior art teachings and prepared heat shrinkable biaxially oriented three layer thermoplastic films with a small concentration such as 0.1 wt. % hydrotalcite in the vinylidene chloride copolymer barrier layer of a biaxially oriented heat shrinkable multilayer with EVA inner and outer layers. This was partially successful in reducing the film odor after irradiation at 3-4 MR. However, there was still a noticeable odor in the irradiated film. In an attempt for further improvement, the hydrotalcite loading was further increased to 0.3 ppm in the vinylidene chloride copolymer barrier layer. There was further reduction in the odor but unfortunately the hydrotalcite loading was so high as to cause agglomeration of particles and numerous bubble breaks causing process interruptions during biaxial orientation of the film by the double bubble procedure. It was apparent from these experiments that the irradiated film odor could not be substantially eliminated by hydrotalcite addition to the vinylidene chloride copolymer barrier layer.
Part of this problem is believed due to the fact that the total chemistry of the odor-producing compounds in the vinylidene chloride copolymer barrier layer of an irradiated multilayer film is extremely complex. Between fifty and sixty compounds have been identified as constituents in the odor-generating gas evolved from the irradiated film.
An object of the invention is to provide an improved irradiated multilayer thermoplastic film having a vinylidene chloride copolymer type barrier-core layer, which is characterized by substantially lower odor than heretofore achieved with comparable prior art films.
Another object is to provide an improved method for manufacturing a heat shrinkable biaxially oriented irradiated multilayer film having a vinylidene chloride copolymer barrier layer, which film is characterized by substantially lower odor than corresponding films prepared by prior art techniques.
Still another object is to provide a biaxially oriented and heat-shrinkable thermoplastic bag formed of irradiated multilayer film having very mild (if any) odor, and comprising a vinylidene chloride copolymer barrier layer and polyolefin inner and outer layers, the three layers being cross-linked by ionizing irradiation of the entire film.
A further object is to provide a food product package comprising a food body entirely enclosed within a thermoplastic bag formed by irradiated multilayer film as described in the immediately preceding paragraph, the film being heat shrunk around the food body with the inner layer inside surface in contiguous relation therewith, yet with very mild (if any) odor from the bag.
Other objects and advantages of the invention will be apparent from the ensuing disclosure and appended claims. As will be explained hereinafter, this invention achieves all of the aforedescribed objects in an unexpected and inexpensive manner.